Great theory! The quote fom Chosen is especially intriguing - but if Buffy and Faith both had to die for the next Slayer to be called, wouldn't that mean that one the power had been shared amongst the Potentials, all of them would have to die for the next Slayer to be called? Rather a bugger for future generations...

That fits with the Fray comic in which there have been no Slayers for centuries - in fact my hypothesis is that Willow has mortgaged the future by Calling all the Potentials, and there will be no more Slayers for a number of generations equal to the number of Called potentials.